The current state of the art for Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access (E-UTRAN) is captured in 3GPP TS 36.300; Overall description; Stage 2. The operation of Uplink Layer 2 Hybrid Automatic Repeat Request, hereinafter referred to as HARQ, processing is described in 3GPP TS 36.321 Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification, and is summarised below:
The User Equipment, sometimes called mobile station and hereinafter referred to as UE, shall for each Transmission Time Interval, hereinafter referred to as TTI:
if an uplink grant for this TTI has been received on the physical downlink control channel PDCCH; or
if an uplink grant for this TTI has been received in a Random Access Response:                indicate a valid uplink grant and the associated HARQ information to the HARQ entity for this TTI;        
else, if an uplink grant for this TTI has been configured and an uplink grant for this TTI has not been received on the PDCCH nor in a Random Access Response:                indicate an uplink grant, valid for new transmission, and the associated HARQ information to the HARQ entity for this TTI.Please note that the period of configured uplink grants is expressed in TTIs.        
There is one HARQ entity at the UE. A number of parallel HARQ processes are used in the UE to support the HARQ entity, allowing transmissions to take place continuously while waiting for the feedback on the successful or unsuccessful reception of previous transmissions.
At a given TTI, if an uplink grant is indicated for the TTI, the HARQ entity identifies the HARQ process for which a transmission should take place. It also routes the receiver feedback (acknowledgement/negative acknowledgement ACK/NACK information) from the E-UTRAN NodeB (i.e. the base station, hereinafter referred to as eNB), relayed by the physical layer, to the appropriate HARQ process.
At the given TTI, the HARQ entity shall:
if an uplink grant, indicating a new transmission, is indicated for this TTI:                notify the “uplink prioritisation” entity that the TTI is available for a new transmission;        if the “uplink prioritisation” entity indicates the need for a new transmission:                    obtain the MAC Packet Data Unit PDU to transmit from the “Multiplexing and assembly” entity;            instruct the HARQ process corresponding to this TTI to trigger the transmission of this new payload using the identified parameters.                        else:                    flush the HARQ buffer.                        
else:                if an uplink grant, indicating a re-transmission, is indicated for this TTI; or        if the HARQ buffer of the HARQ process corresponding to this TTI is not empty:                    instruct the HARQ process to generate a re-transmission.Please note that adaptive retransmissions are ‘sticky’; i.e., when parameters are modified for a retransmission, previous parameters no longer apply for subsequent retransmissions.                        
The UE receives feedback information (ACK/NACK) on the Physical Hybrid ARQ Indicator Channel, hereinafter referred to as PHICH. This information is relayed by the physical layer of the UE to the appropriate HARQ process and handled in combination with the PDCCH Uplink transmission-resource grant information as shown below.
UE detects PHICHUE detects PDCCHindicating theindicating thefollowing:following:UE behaviour:1ACK/NACKTransmissionstarts new transmissionaccording to PDCCH2ACK/NACKRetransmissionretransmits accordingto PDCCH3ACKNoneno retransmission keepsdata in buffer orclear buffer (FFS)4NAKNonenon-adaptiveretransmission
Furthermore it has been agreed that if the UE receives ACK on the PHICH, and the UE detects PDCCH asking for retransmission, the UE behaviour gives precedence to the request for a retransmission and therefore the UE retransmits.
Typically, the UL grants transmitted on the PDCCH are protected by a Cycle Redundancy Check (CRC), and therefore the probability of erroneous decoding is negligible, although the probability that the UE fails to detect the PDCCH message may be as high as 10−2. The ACK/NACK transmissions on the PHICH channel are not CRC-protected and are typically transmitted with an error rate of 10−3 to 10−4.
A mobile terminal can operate by decoding PDCCH then decoding PHICH, or by decoding PHICH then PDCCH if this is required.
FIG. 1 shows the UE processing of the PDCCH and PHICH where the PDCCH is decoded first.
In more detail, the UE looks for a valid PDCCH with an UL grant. If the PDCCH has been correctly decoded, then the UE looks for ACK/NACK on PHICH. If ACK or NACK is received, then the UE looks at the Incremental Redundancy Version (IRV) indicator in the PDCCH message. The UE derives from the IRV indicator whether the eNB requires a retransmission of the previous packet or a new transmission. Then, either an adaptive retransmission of the previous packet or a new transmission is carried out. Then, a valid UL grant is sent. An adaptive retransmission uses uplink transmission resources which are indicated explicitly by the PDCCH grant and are therefore not necessarily the same as for the previous transmission.
If the PDCCH failed to be decoded, then the UE looks for ACK/NACK on PHICH. If ACK is decoded, the UE assumes the previous packet was received correctly and no further UE action is carried out. If NACK is decoded, the UE assumes the need for non-adaptive retransmission (i.e. using the same uplink transmission resources as for the previous transmission). There is a possibility of collision of uplink transmissions if there is a missed PDCCH grant, as the UE would assume that the retransmission is to be non-adaptive and therefore reuse the same uplink transmission resources as for the previous transmission whereas in fact these resources might have been reassigned to a different UE. Subsequently, a valid UL grant is sent.
FIG. 2 shows the state of the art eNB processing for the UL transmissions.
In more detail, in a first step, the eNB receives an uplink packet. Then, it decodes said uplink packet.
If the decoding has been successful, the eNB sends an ACK on the PHICH. Then it checks if the UE has more data. If this is the case, the eNB sends an uplink grant on the PDCCH; if not then the process is terminated.
If the decoding has failed, the eNB sends a NACK on the PHICH and decides if the retransmission shall be adaptive or non-adaptive. If the retransmission is to be adaptive, the eNB sends an uplink grant on the PDCCH. If the retransmission is non-adaptive, the eNB does not send an uplink grant on PDCCH.